1. Field of the Invention
This invention relates to a transfer device and an image forming apparatus, such as a copier, a printer, etc., incorporating the transfer device.
2. Description of the Background Art
In a conventional image forming apparatus employing an electro-photographic system, an electrostatic latent image is formed in accordance with optical image information on an image bearing member uniformly charged previously and is developed into a visible toner image by toner supplied from a developing device. The visible image is then transferred and fixed onto a recording sheet thereby completing image formation.
However, the surface of the recording sheet bears asperities and is more or less uneven. Accordingly, less toner is transferred onto concavities in the surface of the recording medium than onto convexities therein. Especially, when the recording sheet is very rough, that is, exhibits substantial differences between concavity and convex portions, the toner is not transferred onto the concavities and generates dropouts in the output image as a result.
As a solution to the above-described problem, a transfer rate e of toner can be upgraded by using a transfer bias consisting not of a direct current voltage alone but instead one created by superimposing an alternating current voltage on a direct current voltage as disclosed in Japanese Patent Application Publication Nos. 2006-267486 (JP-2006-267486-A) and 2008-58585 (JP-2008-58585-A). Specifically, the technology disclosed in JP-2006-267486-A utilizes a transfer bias created by superimposing the alternating current voltage on the direct current voltage, and charges a surface of the recording sheet to an opposite polarity to that of toner in accordance with the unevenness of the surface of the recording sheet to transfer the toner onto concavities. Similarly, the technology disclosed in JP-2008-58585-A also utilizes a transfer bias created by superimposing the alternating current voltage on the direct current voltage so that a P-P (Peak to Peak) value of the alternating current voltage is less than twice of the direct current voltage.
However, using a transfer bias created by superimposing the alternating current voltage on the direct current voltage creates more corona products, such as nitrogen oxides, ozone, etc., generated in a discharge process increase when the transfer bias created by superimposing the alternating current voltage on the direct current voltage is used than when only the direct current voltage is used as the transfer bias, because reverse discharging occurs between an intermediate transferring member and a transferring member, and the frequency of discharging times is much greater than when only direct current is used. As the corona products build up inside the image formation apparatus, highly concentrated ozone causes cracks in and accelerates degradation of transferring members (e.g., rubber) shortening the life of constituent parts of the apparatus. Further, nitrogen oxide reacts with moisture in the air and with metal or the like generating nitric acid and metal nitric acid, respectively. These products have a high electrical resistance in low-humidity environments, but react with water in the air and have a low resistance at high temperatures. Thus, these products form a thin film on the surface of the transferring members, and transfer current concentrates at the thin film, resulting in output of abnormal images, i.e., images in which white lines etc., appear.
When a cleaner is used to clean a surface of the transferring member, cleaning performance significantly deteriorates in a portion where the thin film of the above-described products is formed. Specifically, toner adhered to the surface of the transferring member cannot be removed therefrom, thereby causing a back side stain on a transfer medium as a problem.
Further, use of a cleaner to clean the surface of the image bearing member causes a significant deterioration in cleaning performance significantly deteriorates at portions where the above-described corona products is formed. Further, toner adhering to the surface of the image bearing member cannot be removed therefrom and thus soils the transfer recording medium as a result. The same problem arises with a use of a cleaner to clean the surface of the image bearing member.